Beam traveling wave amplifier tube



p 1953 A. q. CLAVIER 2,654,047

BEAM TRAVELING WAVE AMPLIFIER TUBE Filed Jan. 20. 1948 2 Sheets-Sheet l IN V EN TOR. fl/VD/Pf G. 624 VIE/P ATTORNE Y A. G. CLAVIER .BEAM TRAVELING WAVE AMPLIFIER TUBE Sept. 29, 1953 2 Sheets-Sheet 2 Filed Jan. 20. 1948 Mm l.

INVENTOR. I KINORE'G. CZflV/fl? NN w ATTORNEY Patented Sept. 29, 1953 BEAM TRAVELING WAVE AMPLIFIER TUBE Andr Gabriel Clavier, New York, N. Y., assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application January 20, 1948, Serial N 0. 3,325

1'7 Claims. 1

The present invention relates to travelling wave amplifiers and more particularly to travelling wave amplifiers with a large amplification factor and with large power output.

As known in the art today travelling wave amplifiers utilize reduced-velocity-wave guides for propagation of an electromagnetic wave with a longitudinal electric field component along an electron beam path. Such a tube is limited in power output. The most common type of reduced-velocity-wave guide which can be excited in a mode with a longitudinal electric field component is one made from a helix of wire and a travelling wave amplifier tube using a helix is limited in power output fundamentally because of the size of the helix. This limits the size of the beam current and hence the kinetic energy of the beam which is equal to the product of the beam current and the beam voltage and which is directly proportional to the power output of the tube. Also the length of helix is limited and hence the amplification.

Furthermore, improvement is difilcult since calculations are extremely complex for the reduced-velocity-wave guide. The travelling wave amplifier tends to oscillate and avoidance of this is difficult when methods of damping feedback are determined from calculations based on the wave guide formulae for the slowed-up wave guide.

It is an object of my invention to provide a travelling wave amplifier tube in which a transmission line is provided for propagation of the wave along the beam path.

It is a further object of my invention to provide efiicient coupling means between the beam of electrons which are propagated outside the space between the two conductors which make up the transmission line and the electromagnetic wave which is propagated in the transmission line mode in the space between the two conductors.

It is a further object of my invention to provide a transmission line for use in the travelling wave amplifier, down which line electromagnetic waves are propagated with a velocity comparable to attainable electron velocities.

As a result of the use of a transmission line in the place of a reduced-velocity-wave guide in a travelling wave amplifier, I find that the calculations of the operation of the device are much simpler and that it is possible to obtain a better determination of the undesirable modes of propagation of electromagnetic waves down the transmission line, which waves must be damped out.

Also it is possible to make the transmission line much longer than the reduced-velocity-wave guide or helix of wire. Thus a larger amplification factor is obtained. The transmission line may also be built much larger than the reducedvelocity-wave guide and hence the beam current may be made larger and more power output may be obtained.

In the transmission line mode of propagation there is no low frequency cut off and hence a wide band response may be obtained.

The above-mentioned objects and features will be more readily understood with reference to the following description of a specific embodiment and with reference to the attached drawings of electron beam and the'conductor adjacent the beam is arranged to permit inductive coupling between the transmission line and the beam, While the velocity of propagation of the wave along the line is reduced by suitable means, such as by material having a high dielectric constant arranged between the conductors of the line or by a helix of wire substituted for one of the conductors of the line.

The analysis of the travelling wave amplifier tube from the standpoint that the electromagnetic wave travels along the beam path in a transmission line mode of propagation is quite different from and much simpler than the analysis made on the basis of the wave equations for an electromagnetic wave guide. It starts from a schematic description of the circuits of the travelling wave tube which assumes mutual inductive coupling between a beam current and current flowing in a transmission line.

Reference may here be had to Fig. 1 in which an envelope l encloses an electron gun 2, a focusing and accelerating electrode 3, and a collector electrode 4 defining a beam path therebetween. A coaxial transmission line comprising an outer conductor 5 and a tubular inner conductor 6 is arranged with the inner conductor coaxially about the beam path.

Mutual inductive coupling M is assumed between the current flowing in the transmission line parallel to the beam path and the beam current just as in the case of two parallel conductors. The line equations are as follows:

. -l oX oi av Ow bi where V is the difference of potential in the line cross section, i and i are the variable components of the line and the beam currents respectively, L and C are distributed constants of the line, and M is the mutual inductance between the line and beam currents. A lossless line is assumed. In order to obtain the beam equations small signal theory is assumed; that with the electron velocity 1) having a steady component to and a variable component "01 and the electron density of the beam having a steady component p and a variable component p1, the product plUl is negligible and the electron velocity v is approximately constant. The beam equations are as follows:

and a propagation constant P. When the lines constants L and- C are adjusted so that the phase velocity up of the wave is made approximately equal to no, then and the solution to Equations 1 and 2 simplifies to v 3 E 2 a (1 1 (1 P m M cva In the case where the phase velocity up of the wave propagated down the transmission line is approximately the velocity on of the electrons in the beam, the propagation constant may be written as follows:

where a is the attenuation constant, and B is the phase constant, and e is an arbitrarily small constant.

The positive sign in Equation 4 corresponds to forward waves. When this expression for the propagation constant is substituted in Equation 3, three solutions are obtained for a and e. Equation 3 can be written approximately as follows:

Setting the rational and irrational parts of Equation 5 equal to zero two equations 101' e and. 1

4 are obtained for which there are three solutions.

(5 a=O e 0 (5b) a= o e 0 (50) a--- E63 has a phase velocity smaller than the electron velocity and is amplified.

There -'is also one backward wave corresponding to the expression for the propagation 'con- "stant with the negative sign. This wave has a phase velocity greater than the electron Velocity and is not amplified.

The amplification factor a for the foreward wave of Equation 5c'is as follows:

or substituting' where I0 is the current of the electron beam directed coaxially through the inner coaxial conductor 6 of the transmission line, Z0 is the characteristic impedance of the line, and )t the wavelength along the transmission line.

The results are similar to those obtained in calculations of the operation of a travelling wave amplifier tube based on the wave equations, that is, three foreward waves and one backward wave. The expression for the amplification factor a given in Equation 6a. leads to possible imp-rovement of the travelling Wave amplifier. In particular it is desirable to make In large.

.Also the transmission line may be constructed so that the expression is a-maximum. An expression for may be obtained in terms of the radii of the inner and outer conductors a and b of the transmission line and this is as follows for the case in which the phase velocity on for the electromagnetic wave propagated along the line is close to the steady component of the electron velocity.

where p and a are the permeability and the dielectric constant for the medium between the coaxial conductors. An expression for the mutual coupling between the current flowing in the transmission line and the electron beam current may be foundby examining the component of the electric field intensity Ea along the axis of the transmission line. If it is assumed that the velocity '00 is much smaller than the velocity of light c, th expression for E2 is as follows:

. 2k 1 1 di ev .a b dt where k is a constant of proportionality and i the axial current in the line. Since an expression for M in terms of a and b is found and also for the ratio If this is differentiated with respect to b and the derivative is set equal to zero, the function is found to be a maximum for the ratio equal to 3.5. Thus, if an electromagnetic wave is propagated down a transmission line, the radii of whose conductors satisfy the condition and if a beam is directed coaxially with and adjacent to the transmission line, the wave propaated a little bit slower than the electrons in the beam will be amplified a maximum amount.

The construction and operation of the tubes of Figs. 1 and 3 are as follows. The electron gun 2 is indirectly heated by a coil 9, leads Iii and I I of said coil being connected across a small portion of a voltage source I2. The electron gun 2 is connected directly to lead I0, lead I being connected to the negative side of the high tension voltage source I2. A lead I3 from the focusing and accelerating electrode 3 is connected to the positive side of the source I2. Thus a beam of electrons is directed through inner conductor 6 of the transmission line 5, 6 with a velocity Do. A lead I 4 from the collector electrode 4 is connected to a low voltage point of said source I2 so that the unconverted portion of the kinetic energy of the beam does not necessarily appear as dissipated power at the collector electrode. A lead I5 from the inner conductor is connected to the positive end of the high tension source I2.

In the tube shown in Fig. 1 the velocity of the wave propagated down the transmission line is retarded by a material I of high dielectric constant placed between the conductors 5 and 6. This may conveniently be the same material as that of the envelope I of the device and in this case it is formed integrally therewith, the envelope I being shaped to fit closely between the conductors 5 and 5.

In the tube shown in Figure 3 the velocity of the wave propagated down the transmission line 6, 8 is reduced by using for the outer conductor 8 a closely wound helix of wire.

At the end adjacent the electron gun 2 and at the end adjacent the collector electrode 4 of both constructions the transmission line 5, 6 in Fig. 1 and 6, 8 in Fig. 2 is connected to transverse coaxial conductors I53, I! and I8, IS, the inner conductors I6 and I8 being sealed through the envelope 2. Quarter wave length short circuited coaxial lines 20 and 2I are arranged for matching the impedances of lines I6, I! and I8, I9 to that 6 of lines 5, G in Fig. 1 and 6, B in Fig. 2. Also quarter wave length open lines 22 and 23 are arranged to prevent radiation from the ends of the line.

Thus, a wave is propagated down the transmission line from the end adjacent the electron gun 2 to the end adjacent the collector electrode 4 at a velocity slightly less than the velocity no of the electrons in the beam. The inner conductor 6 of both structures of Figs. 1 and 2 is constructed of parallel wires and inside this conductor there occurs a component of the electric field of the wave parallel to the beam current path. The wave and the beam interact in a manner similar to the interaction that occurs in an ordinary travelling wave amplifier tube in which the wave is propagated along the beam path through a slowed-up wave guide. The wave is amplified.

A solenoid 24 is provided around the tube in both cases for focusing the beam through the inner conductor 6.

A third embodiment of my invention is shown in Fig. 4. In this embodiment the beam is pro- J'ected along a. transmission line 25, 26 outside of the outer conductor thereof. For this purpose a cylindrical gun 21 is arranged outside of and coaxially surrounding conductor 26 and a cylindrical focusing electrode 28 is arranged coaxially around the gun 21. A collector electrode 29 is arranged at the end of the beam path coaxially about the transmission line and between the accelerating and the collecting electrodes 28 and 29 there is arranged a metal cylinder 30 coaxially about the transmission line 25, 26 for shielding the envelope I from the beam.

The outer conductor 26 of transmission line 25, 26 is composed of a plurality of parallel wires and thus mutual inductive coupling M between the wave propagated down the transmission line and the beam is obtained. The inner conductor 26 is a plain solid conductor.

The transmission line 25, 26 is extended through the envelope I of the tube at both ends and thus there are provided means for connecting input and output transmission lines. The wave propagated down the transmission line is slowed up by placing a material 3| of high dielectric constant between the conductors 25 and 26.

In other respects the operation of the tube is similar to that of the tubes of Figs. 1 and 3 and corresponding parts are numbered accordingly.

The squirrel cage-type of conductor is one form only which will give a large mutual inductive coupling between the wave propagated down the transmission line and the beam focused along the line outside the space between the conductors of the line. Other constructions will occur to those skilled in the art. Also other means for reducing the velocity of the wave propagated down the transmission line may be used. While I have described my invention with reference to a particular embodiment, it is not intended to be limited thereto, but only as defined in the appended claims.

What I claim as my invention is:

1. A travelling wave amplifier tube comprising an electron gun and a collector electrode defining a beam path therebetween, a transmission line including two conductors arranged adjacent to and along said beam path for propagating an electromagnetic wave along said beam path in the direction from said gun to said collector electrode, the space between said conductors being separated from said beam path by one of said conductors, said one conductor being subdivided and aesaou disposed, closely adjacent said beam path to provide large mutual inductive coupling between the electromagnetic Wave and the electron beam, means coupled to said transmission line towards the end adjacent said gun for directly applying the electromagnetic wave to said line and means coupled to said line at the other end for removing the electromagnetic wave, said transmission line including means for causing the wave to propagate more slowly than electrons in said beam.

2. A travelling wave amplifier tube comprising an electron gun and a collector electrode defining a beam path therebetween, a transmission line including two coaxial conductors arranged parallel to the beam path for propagating an electromagnetic wave along said beam path in the direction from said gun to said collector electrode, the space between said conductors being separated from said beam path by one of said conductors the one of said coaxial conductor adjacent said beam path being subdivided to provide mutual inductive coupling between the Wave and the beam, means coupled to said line towards the end adjacent said gun for directly applying the electromagnetic wave to said line, and means coupled to said line towards the other end for abstracting energy from said electromagnetic wave said transmission line being adapted to propagate the wave more slowly than electrons in said beam.

3. A travelling wave amplifier tube comprising an electron gun and a collector electrode defining a beam path therebetween, a transmission line including two coaxial conductors arranged coaxially about the beam path for propagating an electromagnetic wave along said beam path in the direction from said gun to said collector electrode, the inner of said coaxial conductors being arranged about said beam path and being adapted to provide mutual inductive coupling between the wave and the beam, means coupled to said line towards the end adjacent said gun for directly applying the electromagnetic wave to said line and means coupled to said line at the other end for removing the electromagnetic wave from said line said transmission line including means for causing the wave to propagate more slowly than electrons in said beam.

4. A travelling wave amplifier tube according to claim 3 in which the inner conductor comprises a plurality of parallel wires arranged coaxially about the beam path.

5. A travelling wave amplifier tube according to claim 3 in which the last-mentioned means comprises a material of high dielectric constant placed between the inner and the outer conductors.

6. A travelling wave amplifier tube comprising an electron gun and a collector electrode defining an electron beam path, therebetween, an envelope arranged about said gun and said collector electrode and said beani path, a transmission line including two coaxial conductors arranged coaxially about the beam path for propagating an electromagnetic wave along said beam path in the direction from said gun to said collector electrode, the inner of said coaxial conductors including a. plurality of parallel wires arranged coaxially about said beam path, means coupled to said transmission line at the end adjacent said gun for directly applying the electromagnetic wave to said transmission line, means coupled to said transmission line at the end adjacent said collector electrode for removing the electromagnetic wave from said transmission line, and a material 8. of high dielectric constant placed between said inner and outer conductors for causing thewave to propagate along said transmission line more slowly than electrons in said beam.

'7. A travelling wave amplifier tube according to claim 6 in which said material of high dielec trio constant is formed integrally with said envelope.

3. An electron discharge device according to claim 6 in which the ratio of the radius of the outer coaxial conductor to that of the inner is approximately 3.5;

9. A travelling wave amplifier tube comprising an electron gun and a collector electrode defining :a beam path therebetween, and a transmission line including two coaxial conductors arranged coaxially about the'beam path for propagating an electromagnetic Wave along said beam path in the direction from said gun to said collector electrode, the inner of said coaxial conductors being arranged about said beam path and being adapted to provide mutual inductive coupling between the wave and the beam, means coupled to said transmission line at the end adjacent said gun for directly applying the electromagnetic wave to said transmission line, means coupled to said transmission line at the end adjacent said collector electrode for removing the electromagnetic wave from said transmission line, said transmission line being adapted to propagate the wave more slowly than electrons in said beam.

10. A travelling wave amplifier tube according to claim 9 in which the outer conductor of said transmission line comprises a helix of wire whereby the wave transmitted along said transmission line is propagated more slowly than electrons in said beam.

11. A travelling wave amplifier tube comprising an electron gun and a collector electrode defining a beam path therebetween, an envelope enclosing said gun and collector electrode and arranged coaxially about said beam path, a transmission line including two coaxial conductors arranged coaxially about the beam path for propagating an electromagnetic wave along said beam path in the direction from said gun to said collector electrode, the inner of said coaxial conductors including a plurality of parallel conductor wires arranged coaxially about said beam path inside said envelope for providing mutual inductive coupling between the wave and the beam, the outer of said coaxial conductors comp-rising a helix of wire arranged about said envelope, means coupled to said transmission line at the end adjacent said gun for directly applying the electromagnetic wave to said transmission line, and means coupled to said transmission line at the end adjacent said collector electrode for removing the electromagnetic wave from said transmission lines 12. A travelling wave amplifier tube comprising a cylindrical electron gun and a collector electrode defining a cylindrical electron beam path therebetween, a transmission line including two coaxial conductors arranged coaxially inside said beam path for propagating. an electromagnetic wave along said beam path in the direction from said gun to said collector electrode, the outer of said coaxial conductors being subdivided and disposed closely adjacent said beam path to provide mutual inductive coupling between the wave and the beam, means coupled to said transmission line at the end adjacent said gun for directly applying the electromagnetic wave to said transmission line, and means coupled to said transmission line at the end adjacent said collector electrode for removing the electromagnetic wave from said transmission line, said transmission line including means for causing the wave to propagate more slowly than electrons in said beam.

13. A travelling wave amplifier tube according to claim 12 in which said outer conductor comprises a plurality of parallel wires arranged coaxially inside said beam path.

14. A travelling wave amplifier tube according to claim 13 in which said means for causing the wave to propagate slowly comprises a material of high dielectric constant arranged between the coaxial conductors.

15. A travelling wave amplifier tube according to claim 14 in which the ratio of the radius of the outer coaxial conductor to that of the inner coaxial conductor is approximately 3.5.

16. A travelling wave amplifier tube comprising a cylindrical electron gun and a collector electrode defining an electron beam path therebetween, an envelope enclosing said electron gun and collector electrode and beam path, a transmission line including two coaxial conductors arranged coaxially inside said beam path for propagating an electromagnetic wave along said beam path in the direction from said gun to said collector electrode, the outer of said coaxial conductors including a plurality of parallel wires arranged coaxially inside said beam path for providing mutual inductive coupling between the Wave and the beam, means associated with said transmission line at the end adjacent said gun for ing the electromagnetic wave each comprises an extension of said transmission line through said envelope one at the end adjacent said electron gun and the other at the end adjacent said collector electrode.

ANDRE GABRIEL CLAVIER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,122,538 Potter July 5, 1938 2,233,126 I-Iaeff Feb. 25, 1941 2,300,052 Lindenblad Oct. 27, 1942 2,367,295 Llewellyn Jan. 16, 1945 2,511,407 Kleen et al June 13, 1950 2,541,843 Tiley Feb. 13, 1951 2,578,434 Lindenblad Dec. 11, 1951 OTHER REFERENCES Article by Kompfner, Proo. I. R. E. for February 1947, pp. 124-127, inclusive. (Copy in Div. 54.) 

